New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31: 693-700
0028-8330/97/3105-0693 $7.00/0 © The Royal Society of New Zealand 1997
693
Evaluating techniques for sampling stream crayfish
{Paranephrops planifrons)
CHARLES F. RABENI
Missouri Cooperative Fish and Wildlife
Research Unit
Biological Resources Division
U.S. Geological Survey
112 Stephens Hall, University of Missouri
Columbia, Missouri 65211, United States
captured more large crayfish relative to
electrofishing or the quadrat sampler.
Keywords crayfish; population structure;
depletion; mark-recapture; electrofishing; New
Zealand
INTRODUCTION
KEVIN J. COLLIER
STEPHANIE M. PARKYN
National Institute of Water & Atmospheric
Research Ltd
P.O.Box 11 115
Hamilton, New Zealand
BRENDAN J. HICKS
Department of Biological Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract We evaluated several capture and
analysis techniques for estimating abundance and
size structure of freshwater crayfish (Paranephrops
planifrons) (koura) from a forested North Island,
New Zealand stream to provide a methodological
basis for future population studies. Direct
observation at night and collecting with baited traps
were not considered useful. A quadrat sampler was
highly biased toward collecting small individuals.
Handnetting at night and estimating abundances
using the depletion method were not as efficient as
handnetting on different dates and analysing by a
mark-recapture technique. Electrofishing was
effective in collecting koura from different habitats
and resulted in the highest abundance estimates, and
mark-recapture estimates appeared to be more
precise than depletion estimates, especially if
multiple recaptures were made. Handnetting
M97007
Received 11 March 1997; accepted 21 July 1977
Recent studies have shown that freshwater crayfish
can influence stream ecosystem processes such as
sediment accumulation and organic matter
processing and invertebrate community structure
(e.g., Huryn & Wallace 1987; Creed 1994; Parkyn
et al. 1997). Although the ecological importance of
crayfish in aquatic ecosystems is becoming more
evident (Lodge & Hill 1994; Momot 1995), our
ability to adequately assess important population
characteristics, e.g., density or size structure,
remains suspect. Stream-dwelling crayfish have
been sampled in a variety of ways, and biases are
known for some techniques but not for all
(DiStefano 1993). For example, baited traps are
widely used for many North American genera—
Orconectes, Cambarus, and Pasifasticus—but
favour the collection of larger males in the
population (Mason 1975; Capelli & Magnuson
1983; Olsen et al. 1991). Electrofishing may be
inappropriate for areas with heavy brush cover
where crayfish are inaccessible (Westman et al.
1978), whereas hand collections are restricted to
situations of good visibility and may be subject to
variable escapement (Roell & Orth 1992). Seining
is restricted to habitats without obstructions (Brant
1974), as are quadrat benthic samplers (Rabeni
1985).
Because accurate determination of population
characteristics is a requirement for many ecological
studies on crayfish, it is important to know the
limitations of sampling methods. In New Zealand,
published work on stream-dwelling crayfish
Paranephrops spp. (koura) has focused on growth,
breeding, and behaviour (Hopkins 1966, 1967;
Shave et al. 1994) where both handnetting and
electrofishing sampling were used but for objectives
694
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31
where sampling biases were not important. Little
is known of the abundance or population characteristics of lotic freshwater koura, perhaps partly
because they are not easily collected using
conventional benthic invertebrate sampling
techniques.
The objective of this study was to assess the
relative usefulness of a number of sampling
techniques in estimating the population size
structure and density of a population of P. planifrons
from a North Island, New Zealand stream. This
study is unique because it estimates population
characteristics of crayfish in the same system using
different methods.
STUDY AREA
The study area was on Firewood Creek located in
the Ngaruawahia Forest northwest of Hamilton,
North Island, New Zealand (NZMS260 map
reference S14 976888). The site was 40 m a.s.l. and
had an up-stream catchment area of 2.01 km2. The
site was heavily shaded by vegetation (diffuse noninterceptance radiation = 0.016), and mean water
temperature over the early summer (NovemberDecember) averages 12.5°C (Smith et al. 1993;
NIWA unpubl. data). Riparian vegetation was
predominantly nikau (Rhopalostylis sapida), ferns
(Dicksonia, Cyathea, and Blechnum spp.), and
parataniwha {Elatostema rugosum) beneath a mixed
forest overstory which included tawa (Beilschmieda
tawa), rimu (Dacrydium cupressinium), and
rewarewa (Knightia excelsa).
The study site consisted of three contiguous
reaches: an up-stream reach c. 17 m in length, a
middle section 14 m long, and a lower section 11
m long. The site was chosen because of its physical
complexity and was composed on an areal basis of
c. 21% dammed pool, 20% lateral scour pool, 4%
undercut bank, 28% run, 24% riffle, and 4% side
channel (see Bisson et al. 1982 for definitions of
habitats). The site averaged 2.9 m in width (SD =
0.9) with a range of 0.9^4.6 m. Current velocities
at the water surface, taken during quadrat sampling,
ranged from <10 to a little over 30 cm s"1. Depths
ranged from a few centimetres in riffles to c. 40 cm
in pools.
with multiple passes through each reach. We were
able to estimate abundance by direct count (both
observation and handnetting), depletion, and single
and multiple recapture methods. Size-frequency
comparisons among methods were used to examine
equipment biases, and we examined overall differences in the size distribution of koura caught by
handnetting, electrofishing, and quadrat sampling.
Abundance estimates were made by direct
observation and handnetting on consecutive
evenings (between 2100 and 0100 h NZST) on 30
and 31 January 1996. Handnetting consisted of
direct stalking of koura during darkness with the
aid of hand-held torches and helmet-mounted lights.
Generally, two people would start at the downstream end of each of the three reaches and work
slowly up-stream for c. 20 min, netting all observed
koura. This was sufficient time to carefully examine
all parts of the reach, and was termed a "pass". After
c. 10 min the next pass was begun, which allowed
c. 30 min between observations at any one place in
the stream. Generally, three passes were conducted
in each reach. Direct observation was the number
of koura observed on the first pass of the
handnetting operation, as observing during multiple
runs would undoubtedly recount individuals. The
observation period was c. 2 min m~2 of substrate
surface, and was considered more than sufficient
to detect any exposed crayfish.
Electrofishing consisted of using a back-pack
electrofishing unit, battery operated on 1 February
and gasoline powered on 22 February, to slowly
examine all of the wetted perimeter during daytime
Table 1 Schedule of sampling, sampling method, and
type of analysis used to estimate abundance of stream
crayfish {Paranephrops planifrons) (koura) in 1996.
Date
30 Jan
31 Jan
Method
Analysis
Handnetting
Observation
Handnetting
Depletion, direct count
1 Feb
Observation
Electrofishing
1 Feb
Handnetting
METHODS
5 Feb
Quadrat sampler
The experimental design allowed for a variety of
sampling techniques and analyses. Three
contiguous reaches were sampled on all but one date
22 Feb
Electrofishingt
fMiddle reach only.
Depletion, markrecapture
Depletion, multiple
mark-recapture
Check on electrofishing
Direct estimates
Depletion, multiple
mark-recapture
Rabeni et al.—Sampling stream crayfish
(Table 1). A shocker and a netter would work
together, while directly down stream a person with
a small seine captured any drifting koura. On 1
February, two electrofishing passes of the entire
study site were made, whereas on 22 February five
passes of the middle reach only were made to
evaluate the precision of the depletion electrofishing
technique. Koura collected on 1 February were held
outside the stream in buckets, and on the same
evening handnetting similar to that conducted on
30 and 31 January was done to obtain a conservative estimate of crayfish not collected by
electrofishing.
A quadrat sampler was used to obtain direct
estimates of koura abundance in the substrate. The
sampler consisted of a metal frame enclosing a
0.5 m2 area (1 x 0.5 m rectangle, 0.4 m high) and
had four legs (0.1m long) which were worked into
the substrate. The sampler was surrounded on three
sides by 3-mm netting with the bottom weighted
by a chain, and had a 1-mm mesh net on the downstream end. It was operated in a manner similar to
that of a Surber bottom sampler, where the sampler
was laid on the substrate and the substrate within
disturbed by hand. Koura were carried into the
collection bag by natural or operator-induced
current. Ten quadrat samples were taken during the
daytime of 5 February.
A wire-mesh trap baited with dog biscuits or
sheep liver was tried but found to be ineffectual in
capturing P. planifrons. This method was
abandoned.
Regardless of collection method, collected
individuals were placed in buckets or chilly bins
alongside the stream. Each koura was measured
(from the posterior border of the eye-socket to the
mid-dorsal posterior border of the carapace
(Hopkins 1966), referred to hereafter as carapace
length (CL)) to the nearest millimetre using vernier
calipers. The edge of the telson fin was clipped to
distinguish the date or method of collection and
koura were released near the point of capture. Telson
fins of koura <5 mm CL were too small to be clipped.
Population estimates were calculated three
ways. For estimates from a single visit to a site we
used the removal method which, by using catches
of animals from repeated standardised catch effort,
regresses the catch-per-effort on the cumulative
number of animals removed. The estimated
population size is the cumulative catch for which
the expected catch-per-effort is zero, which is the
x-intercept (Zippin 1958). For two or more sampling
visits to a site capture-recapture methods were used.
695
Captured animals were uniquely marked and on the
second and subsequent collections the proportion
of unmarked animals and the recapture of marked
animals were used to estimate population
abundance. The Chapman modification of the
Peterson Index was used for single recaptures and
the Schnabel method used for multiple recaptures
(Ricker 1975).
RESULTS
Population estimates
Direct observations and handnetting
Direct observation in the three reaches combined
resulted in a total counting of 43 (0.35 koura irr 2 )
individuals the first night and 31 (0.25 trr 2 ) the
second night. Four passes with handnets and
subsequent analysis of all koura collected by the
Zippin depletion method (Zippin 1958) resulted in
abundance estimates of 96 and 89 (0.79 and 0.73
m~2, respectively) koura each night. Both estimates
had impressively tight 95% confidence intervals of
about ±10% of the mean.
Because the 30 January captures were marked
with a telson clip, an abundance estimate of koura
>5 mm CL was possible from the 31 January
collection using mark-recapture analysis. An
estimate of total abundance was also made by
adjusting for small individuals that could not be
marked by using a correction factor of 12% which
was the mean percentage of koura <5 mm from all
collections except the quadrat collections. This
corrected estimate (Fig. 1) does not have an
associated variance.
Using the Chapman modification of the Peterson
method (Ricker 1975) an abundance estimate of 274
koura (>5 mm CL; 2.25 m"2) with a 95% confidence
interval of 175-373 was obtained (Fig. 1). An
estimate of total abundance was also made by
adjusting for small individuals that could not be
marked. This corrected estimate (306; 2.51 m~2)
does not have an associated variance. Thus, the
estimated abundance from direct observation was
about half the number actually collected by
handnetting, and for handnetted crayfish the markrecapture estimate was about two and a half times
greater than the depletion estimate (Fig. 1).
Electrofishing
Two capture runs with the electrofisher on 1
February enabled an analysis by the depletion
method (Zippin 1958), and resulted in an estimate
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31
696
700
Fig. 1 Freshwater crayfish
(Paranephrops
planifrons)
(koura) abundance estimates with
95% confidence intervals for all
sampling dates, methods, and
analyses. Numbers with an (*) are
abundance estimates adjusted for
very small (<5 mm in length)
individuals. Data of 1 February
include koura collected by
handnetting subsequent to
electrofishing. (Dep = depletion
estimate; MR = mark-recapture.)
600
'546
*535
8
500
§400
T3
3 300
•317
200
100
n
10
400—,
Fig. 2 Freshwater crayfish
(Paranephrops
planifrons)
(koura) abundance estimates with
95% confidence intervals from
the middle reach of the study site
using various sampling and
population estimation techniques.
(Dep = depletion estimate; MR =
mark-recapture.)
300—
200X)
<
100-
u
r
I
n
•g
<0
of 400 individuals in the site with fairly wide 95%
confidence intervals of about ±50% of the mean
(Fig. 1).
A mark-recapture abundance estimate was made
on the 1 February electrofishing collection, analysed
by the Schnabel multiple recapture technique
8
(Ricker 1975) using marked individuals from both
handnetting nights. This resulted in an estimate of
471 (3.86 m~2) with a 95% confidence interval of
370-647 (Fig. 1). The 12% correction for very small
crayfish raised the total site estimate to 527
(4.32 m"2).
697
Rabeni et al.—Sampling stream crayfish
Handnetting on the evening of 1 February
resulted in the collection of an additional 30 crayfish
which, by depletion analysis, indicated that 54
animals remained in the stream after electrofishing.
Fifty-four additional crayfish gives a total well
within the previously determined confidence
interval. By incorporating the 1 February
handnetted catch of marked and unmarked
individuals into the mark-recapture calculations the
abundance estimate becomes 461 (3.78 irr 2 ). This
is similar to the estimate when using only
electrofishing recaptures 471 (3.86 nT2) but with a
somewhat tighter 95% confidence interval (Fig. 1).
Quadrat
60-1
Quadrat sampling
This technique yielded a mean of 1.4 individuals
per sample with a standard deviation of 1.3. This
represents a site abundance estimate of 317 with a
95% confidence interval of about ±50% of the
mean.
On 22 February five electrofishing passes were
made on the middle reach (14 m long; 37.4 m2) of
the study area (Fig. 2). The abundance estimate of
79 individuals (2.11 nT2) made by the depletion
method (Zippin 1958) was similarto the 1 February
estimate (68; 1.81 m"2) whereas 22 February
Schnabel mark recapture estimate, using all
previously marked individuals was not very
different from the 1 February Schnabel estimate of
181 (4.83 m"2) but it possessed a smaller 95%
confidence interval (Fig. 2).
Size-frequency analysis
Collections using the quadrat sampler were
dominated by more smaller individuals than were
collections from the other two sampling methods.
Fifty-seven percent of all individuals collected with
this device were in the 3-6 mm CL size range (X=
5.8 mm CL), and none were over 15 mm CL.
The size structures of P. planifrons collected by
electrofishing and handnetting were more similar
to each other than to collections made with the
quadrat sampler (Fig. 3) and were not dominated
by small individuals. Whereas there was an identical
modal size range of 6-9 mm CL, handnetting
captured a relatively greater proportion of mid-sized
individuals (those between 12 and 18 mm CL);
electrofishing captured a greater proportion of
smaller crayfish (<9 mm CL). The mean size
captured by handnetting was 13.6 mm CL, whereas
the mean size captured by electrofishing was
10.4 mm CL.
Handnetting
15-
3
Jill,,.
6
9
12
15
18
21
24
27
30
Length
Fig. 3 Carapace length-frequency distributions (mm)
of freshwater crayfish {Paranephrops planifrons) (koura)
captured by three methods.
The biases resulting in larger crayfish being
captured by handnetting relative to electrofishing
were also evident on 1 February when the site was
electrofished and then handnetted before the
individuals collected by electrofishing were
returned to the stream (Fig. 4). Handnetting again
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31
698
Fig. 4 Carapace lengthfrequency distributions (mm) of
freshwater crayfish (Paranephrops planifrons) (koura)
captured by electrofishing on 1
February (dark bars), and a
subsequent collection by
handnetting (light bars).
3-6
6-9
9-12
12-15 15-18
Length
18-21
captured a relatively greater proportion of larger
koura in the 12 and 15 mm CL size groups. The
mean size collected by electrofishing, 10.4 mm, was
substantially smaller than the handnetted mean size
of 13.0 mm.
What could not be determined was whether the
differences in mean size were a result of handnetting
collecting relatively greater numbers of mid-sized
individuals or relatively fewer smaller individuals,
or whether electrofishing caught relatively greater
numbers of small (6-9 mm CL size class)
individuals.
We investigated whether vulnerability to capture
was related to size when repeated captures were
involved. No evidence of this was found. On 22
February the five electrofishing runs showed some
variation in the mean size of individuals from first
to last pass of 16.6, 13.1, 12.9, 17.4, and 11.1 mm.
The total captures from each of the two
electrofishing runs on 1 February showed little
difference in mean size of individuals between the
first (10.9 mm) and second (9.7 mm) passes.
DISCUSSION
Studies of population dynamics and production of
Paranephrops spp. and other crayfishes in streams
require knowledge of densities and year-class
composition with some degree of confidence. Such
confidence may be difficult to obtain, especially
with relatively low-density populations. As a
general rule, the greater the density or abundance
of the population, the easier it is to achieve a
particular level of precision. Low-density
populations require relatively high captures for
depletion methods and high captures and recaptures
for mark-recapture methods.
Assuming our study site contained 500
individuals (4 n r 2 ) , over three-quarters of the
population needed to be captured to be within 10%
21-24
24-27
27-30
of the true population size with 90% probability
using either removal or mark-recapture methods
(Zippin 1958; Robson & Regier 1964). This level
of precision is probably not realistic for stream
koura regardless of sampling intensity. A more
realistic goal often used in research studies with
invertebrates is a precision of within 20% of the
true population size (Elliott 1971; Cummins 1975).
This would require capturing roughly half of the
population, which appears to be an attainable goal
in situations such as ours if multiple collections are
made.
A consequence of the rather wide variance
associated with our estimates is that it becomes
more difficult to detect statistically significant
differences between populations or in a population
overtime. To detect a significant change in densities
(defined as non-overlapping 95% confidence
intervals) using the multiple mark-recapture
sampling estimate which ended on 1 February
would require a difference in means (increase or
decrease) of c. 60%.
Equally important as precision of abundance
estimates is the question of gear size selectivity.
Length-frequency distributions are commonly used
in studies of crayfish and other decapods to
delineate age groups (France et al. 1991), and errors
at this stage of the study can influence evaluations
of many aspects of population dynamics.
Investigators not able to quantify their populations
often are content to obtain a "representative"
qualitative sample. However, a sampler unable to
collect a quantitative sample probably is also size
selective, so the results of even qualitative samples
should be questioned—especially if they are used
to evaluate population characteristics such as age
distribution (e.g., France et al. 1991) or mortality.
An important assumption for use of markrecapture methods is that immigration or emigration
during the study was negligible (Robson & Regier
Rabeni et al.—Sampling stream crayfish
1964). If random or directional movements occurred
unmarked individuals would enter the study area
while marked and unmarked animals would exit.
The number of marked individuals in the
population, and therefore the percentage of marked
individuals would also decrease. We conclude that
immigration and emigration were not significant
because the percentage of marked individuals
captured as a percentage of total available marked
individuals actually increased over time from the
second to last sampling in both the total site and in
the middle site estimates.
Another possible cause of error is that the
marked individuals were not equally susceptible to
recapture as unmarked individuals (Robson &
Regier 1964). P. planifrons may "learn" to avoid
capture on subsequent exposure to the collector.
Less vulnerable animals will result in
underestimation of abundances using depletion
techniques and overestimation of abundances using
mark-recapture techniques. Similar errors would
result if captured koura suffered some mortality
because of handling and telson clipping, or if
electrofishing amplified the flexing of the telson
muscles of freshly molted crayfish to cause muscle
detachment. However, no mortality was observed
when numerous koura were held in buckets for
several hours after electrofishing and marking on 1
February.
Certainly the effectiveness of any technique is
dependent upon habitat conditions and the
behaviour of the organisms. P. planifrons of our
stream were exposed outside of protective cover
only at night. Depletion estimates may be low
because the entire population was not vulnerable
to capture on any one collection date. Boulders,
undercut banks, and tangles of root material, where
P. planifrons appear to spend extended periods of
time during the day, offer potential refugia from
being collected by some techniques. Only
electrofishing appeared effective in heavy cover,
where increased activity of the affected animals
made them visible and susceptible to netting in
slow-current areas and being carried into a seine in
fast-current areas.
CONCLUSIONS AND
RECOMMENDATIONS
Although the true density and age structure of this
population remains unknown our multiple gear
approach allows us to infer inherent biases of each
method. We captured 296 different individuals >0.5
699
mm CL and at least 40 different smaller koura
giving a total of 343 (2.81 nr 2 ) which is a minimum
estimate of the population. The direct censusing
obviously severely underestimated abundances,
could not be used for size analysis, and should not
be considered further. The method was attempted
because in some situations it is thought to be the
most accurate method to obtain population
information about crayfish (Lamontagne &
Rasmussen 1993). Handnetting depletion gave a
precise (i.e., tight confidence interval) estimate, but
also greatly underestimated abundances. It was
revealing how it was possible to obtain quite small
confidence intervals using a particular technique
even though the technique was highly biased.
Handnetting appears to capture relatively more
larger individuals than any of the other methods;
either larger crayfish are observed more readily, or
larger crayfish are more active and exposed at night.
We recommend handnetting for abundance
estimates only in conjunction with a second capture
method in capture-recapture efforts.
The quadrat sampler captured relatively more
smaller individuals than the other methods.
Although 16% of captures from all other methods
over the course of the study were <5 mm CL, the
quadrat sampler collected 81% of its total in this
approximate size range. Handnetting collected
relatively more larger individuals than the other
methods. Density estimates from quadrat sampling,
although similar to multiple mark recapture
estimates, are not really comparable because the
quadrat sampler captured such a high percentage
of very small individuals. The quadrat sampler can
be used to investigate young-of-year koura in
appropriate habitats, but we do not recommend any
of the techniques for quantitatively sampling very
small individuals throughout a stream.
Electrofishing was considered the most accurate
sampling tool in this study. Electrofishing depletion
gave a 2-4 times greater abundance estimate than
the handnetting method, and was comparable to
mark-recapture estimates using electrofishing as the
recapture technique. Multiple mark-recapture is
preferable to single mark-recapture as it substantially reduces variance. Marking animals
collected by one method and recapturing by another
is preferable to using one method throughout.
ACKNOWLEDGMENTS
Our thanks to David West and Helen McCaughan who
assisted with the electrofishing. Liz Halsey, Paul Johnson,
700
New Zealand Journal of Marine and Freshwater Research, 1997, Vol. 31
and Jane Rabeni assisted with field work. Greg
Whitledge, University of Missouri, assisted with the
analyses. This is a contribution from the Missouri
Cooperative Fish and Wildlife Research Unit (Biological
Resources Division, U.S. Geological Survey; Missouri
Department of Conservation; University of Missouri; and
Wildlife Management Institute cooperating).
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